A power amplification circuit is an important part of a transmitter in a communications system. As shown in FIG. 1, a power amplification circuit mainly includes three parts: an input matching network, a power amplifying transistor, and an output matching network. An existing power amplifying transistor is shown in FIG. 2. An input matching network and an output matching network are connected in the power amplifying transistor by using pins in the package. A specific internal structure of the power amplifying transistor is shown in FIG. 3, including a power amplifying transistor die (which is generally an active component and is a core part for power amplifying) and a metal oxide semiconductor capacitor (Moscap) that are packaged together. In FIG. 3, LB0, LB1, and LB2 are bonding wires (a bonding wire refers to a metal bonding wire that connects two separate components, and is usually a gold bonding wire or an aluminum bonding wire. The bonding wire is often used inside a device and demonstrates an inductance characteristic at a frequency less than 10 GHz) used to connect separate components. The bonding wire LB0 is specifically used to connect a gate of the power amplifying transistor die and an input pin of the power amplifying transistor. The bonding wire LB1 is specifically used to connect a drain of the power amplifying transistor die and the metal oxide semiconductor capacitor (Moscap). The bonding wire LB2 is specifically used to connect the drain of the power amplifying transistor die and an output pin of the power amplifying transistor.
FIG. 4 is an equivalent circuit of an output end of a power amplification circuit that uses the power amplifying transistor shown in FIG. 3, where Ropt is output resistance of the power amplifying transistor die when the power amplifying transistor die is working, and Cds is a parasitic capacitor between the drain and a source of the power amplifying transistor die. A low-frequency resonant circuit of the output end includes the power amplifying transistor, the output matching network, and a grounded back-end network connected to an output end of the output matching network. A low-frequency resonance frequency is determined by inductance and capacitance in the low-frequency resonant circuit of the output end; if the inductance is relatively large, the low-frequency resonance frequency is relatively low.
However, a relatively low low-frequency resonance frequency of a power amplification circuit in a transmitter is undesirable in many communications networks at present. For example, in a 3G (third generation mobile communications technology) network and a 4G (the fourth generation mobile communications technology) network, modulated signals are all broadband signals, and the modulation signals are transmitted by using a power amplification circuit in a transmitter after being converted into radio frequency signals. To ensure that adjacent channel interference in the networks meets a protocol requirement, DPD (digital pre-distortion) correction needs to be performed on a radio frequency power amplification circuit If a low-frequency resonance frequency of the power amplification circuit is relatively low, impedance of an envelope signal of a radio frequency signal varies greatly at an output end of the power amplification circuit, which further causes relatively great changes in characteristics of the power amplification circuit at different moments. Based on a DPD correction principle, the relatively great changes in characteristics of the power amplification circuit at different moments cause a relatively poor DPD correction effect, thereby affecting signal bandwidth that can be supported by the power amplification circuit.
Therefore, how to increase a low-frequency resonance frequency of a power amplification circuit becomes an increasingly hot topic in modern technology research.